Ion sieving

It has been known for some time that polypyrrole, the most studied conducting polymer, exhibits a memory for the anion of the electrolyte used in its preparation [124]. The selectivity of polypyrrole films towards anions is related to the size and 

Biomimetic electrochemical sensors based on molecular imprinting 

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The effect of ion size on the rates of intercalation of NH J, CH3NH+, C2H5NH 3, n-C3H7NHt, i-C3H7NHt, (CH3)2NH , (CH3)3NH+, and (CH3) N+ into Z-4A zeolite has also been investigated using the pressure-jump technique (18). The results shown in Table V illustrate that Z-4A acts like an ion-sieve. For ions with a volume... 

Minerals with stereoselective, catalytic and ion-sieve properties have been discussed. 

Paterson, R. Rahman, H. (1984a) The ion exchange properties of crystalline inorganic oxide-hydroxides. Part II Exclusion of perchlorate from p-FeOOH by an ion sieve mechanism. J. Colloid Interface Sci. 97 423-427... 

For the unique properties of PBs to be exploited, PBs must be deposited properly onto a solid support. It is highly desirable to prepare mechanically robust PBs films with controlled thickness, chemical composition and crystallinity, having ion-sieving membranes and electrochromic devices in mind [6], or to create regular patterns of PB-based single molecule magnets [13],... 

The latter process seems to be very selective against alkali metal cations in the initial stages of exchange and is critically dependent on alkali metal solution phase concentration. Sometimes ion-exchange processes can be described as ion sieving when a particular cation (hydrated or unhydrated) is too large to enter all, or part of, the zeolite internal structure. 

Most zeolites have three-dimensional structures with cavities connected to charmels that exhibit ion sieve properties. If the pore volume of the cavity is large enough, the hydration stripped cations exchanged can be rehydrated in the cavity to simulate their hydration in the solution. The large cations (Rb, Cs, organic cations) cannot enter the channels or windows of the cavities of zeolites with small pores. Table 3 lists the pore openings of some hydrated zeolites [133]. 

The a-zirconium phosphate has a layered structure in which each layer consists of a plane of zirconium atoms coordinated octahedrally to oxygen. The free space in the sides of the layered structure is large enough to allow a spherical ion of 0.263 nm diameter to diffuse the cavity without any obstruction and is accessible to Li, Na" and K" " [139]. However, the size of the window is smaller than the ionic spheres of Rb (0.296 nm) and Cs (0.338 nm) and an ion sieve effect is encountered with these ions. 

Acid salts of Group IV phosphates and arsenates have layered structures with interlayer distances large enough to accommodate a number of ions. The size of the opening to the cavities of the acid salts with the same layered structure is dependent upon the particular composition. Table 5 lists the correlation between ion sieve effects and the interlayer distance of various acid salts [l40j. 

Table 5 Ion Sieve Effect for Alkali Metal Exhibited by Layered Acid Salts...

Ooi, K., Miyai, Y., and Katoh, S., Lithium-ion sieve property of L-type manganese oxide, Solvent Extrac. lonExch., 5, 561, 1987. 

Three properties have made the zeolites of major importance their ion exchange and ion sieve behaviour their selectivity as sorbents and molecular sieves and their catalytic and shape selective catalytic functions. Some dates of significance in their long history are given, with references, in [ 8]. These dates are given below. 

Another significant result from the early work of Barrer [45] was the recognition that Cs (3.30A diameter) could not enter the ANA framework whilst Rb (2.96A) and the smaller alkali metal cations could. This introduced the concept that zeolite internal architectures were capable of selectively removing cations from water on the basis of cation size- the properly described as ion-sieving . 

Larger cations, such as (CH3)4N and CH3CH(NH3)CH2CH3, were completely excluded by ion sieving. 

The concept of ion-sieving originally proposed by R. M. Barrer Q) has not been supported by crystallographic studies of various... 

The authors have estimated the intracrystalline volume of clinoptilolite from the water content of the Na form and conclude from their estimates of the ionic volumes of the organic cations used that 100% exchange is theoretically possible for all ions. They conclude from a consideration of the heulandite structure (41), which is isostructural with clinoptilolite, that partial ion sieving occurs because there are 2 different networks of channels that intersect. Diffusion into and through one network is via rings of 10 tetrahedra having a planar projection that... 

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